A magnetic recording medium, particularly when used for audio cassette tapes, is required to have a good frequency response and good reproduction of original sound because it is used for recording music. Recently such magnetic recording media have also required to have increased runability and durability for use in car stereo, cassette decks and cassette decks provided in radio sets.
A magnetic recording medium for video cassette tapes is used for higher density recording accomplished by shortening the recording wave length of narrowing the track width, and therefore, a magnetic recording medium having higher output, higher S/N ratio and better reproducing ability of original sound is required in this application. With the increase of the portable video tape recorder (VTR), magnetic recording tapes for VTR having much better running durability than conventional tapes are required, while the total thickness of such tapes is thinner, for example, not more than 20 .mu.m.
Consequently, both audio tapes and video tapes currently produced are required to have superior electromagnetic properties, running properties and durability compared to conventional audio tapes and video tapes.
Various additives for magnetic layers have been proposed to provide these qualities, but the additives that are known have not completely satisfied the above requirements.
For example, surface active agent such as fatty acid salts, phosphates, alkyl sulfonates, sulfosuccinates or N-acylsarcosine are used as conventional dispersing agent for ferromagnetic powder as described in, for example, U.S. Pat. Nos. 3,460,984, 4,153,754, 4,285,825, 4,299,852 and 4,394,420.
However, where these conventional dispersing agents are used for preparing magnetic coating compositions having improved dispersibility of ferromagnetic powder, the magnetic layers obtained by coating and drying these magnetic coating compositions on a support often have poor runability at high temperature and high humidity and have poor running durability at normal temperature and humidity.
In addition, various coupling agents such as silane coupling agents or titanium coupling agents as described in, for example, U.S. Pat. Nos. 3,996,407, 4,076,890 and 4,415,630 have been proposed to improve the durability of a magnetic layer by increasing the wetting between the ferromagnetic powder and binder used. However, these coupling agents do not react sufficiently with ferromagnetic powders and deteriorate the dispersibility of ferromagnetic powder to reduce the electromagnetic properties of the resulting tape.
As described above, it is difficult to obtain both adequate elecctromagnetic properties and sufficient durability by using conventional additives for magnetic layers.